help: correct logic error in combining --all and --config

Fix a bug in the --config option that's been there ever since its
introduction in 3ac68a9 (help: add --config to list all available
config, 2018-05-26). Die when --all and --config are combined,
combining them doesn't make sense.

The code for the --config option when combined with an earlier
refactoring done to support the --guide option in
65f9835 (builtin/help.c: add --guide option, 2013-04-02) would
cause us to take the "--all" branch early and ignore the --config
option.

Let's instead list these as incompatible, both in the synopsis and
help output, and enforce it in the code itself.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-help.txt

@@ -11,6 +11,7 @@ SYNOPSIS
 'git help' [-a|--all [--[no-]verbose]]
 	   [[-i|--info] [-m|--man] [-w|--web]] [COMMAND|GUIDE]
 'git help' [-g|--guides]
+'git help' [-c|--config]
 
 DESCRIPTION
 -----------

builtin/help.c

@@ -62,6 +62,7 @@ static const char * const builtin_help_usage[] = {
 	N_("git help [-a|--all] [--[no-]verbose]]\n"
 	   "         [[-i|--info] [-m|--man] [-w|--web]] [<command>]"),
 	N_("git help [-g|--guides]"),
+	N_("git help [-c|--config]"),
 	NULL
 };
 
@@ -553,9 +554,21 @@ int cmd_help(int argc, const char **argv, const char *prefix)
 			builtin_help_usage, 0);
 	parsed_help_format = help_format;
 
+	/* Incompatible options */
+	if (show_all && show_config)
+		usage_msg_opt(_("--config and --all cannot be combined"),
+			      builtin_help_usage, builtin_help_options);
+
+	if (show_config && show_guides)
+		usage_msg_opt(_("--config and --guides cannot be combined"),
+			      builtin_help_usage, builtin_help_options);
+
 	/* Options that take no further arguments */
+	if (argc && show_config)
+		usage_msg_opt(_("--config cannot be combined with command or guide names"),
+			      builtin_help_usage, builtin_help_options);
 	if (argc && show_guides)
-		usage_msg_opt(_("--guides cannot be combined with other options"),
+		usage_msg_opt(_("--guides cannot be combined with command or guide names"),
 			      builtin_help_usage, builtin_help_options);
 
 	if (show_all) {
@@ -570,6 +583,14 @@ int cmd_help(int argc, const char **argv, const char *prefix)
 		list_commands(colopts, &main_cmds, &other_cmds);
 	}
 
+	if (show_guides)
+		list_guides_help();
+
+	if (show_all || show_guides) {
+		printf("%s\n", _(git_more_info_string));
+		return 0;
+	}
+
 	if (show_config) {
 		int for_human = show_config == 1;
 
@@ -583,14 +604,6 @@ int cmd_help(int argc, const char **argv, const char *prefix)
 		return 0;
 	}
 
-	if (show_guides)
-		list_guides_help();
-
-	if (show_all || show_guides) {
-		printf("%s\n", _(git_more_info_string));
-		return 0;
-	}
-
 	if (!argv[0]) {
 		printf(_("usage: %s%s"), _(git_usage_string), "\n\n");
 		list_common_cmds_help();

t/t0012-help.sh

@@ -35,7 +35,8 @@ test_expect_success 'basic help commands' '
 '
 
 test_expect_success 'invalid usage' '
-	test_expect_code 129 git help -g add
+	test_expect_code 129 git help -g add &&
+	test_expect_code 129 git help -a -c
 '
 
 test_expect_success "works for commands and guides by default" '